Massive multiple-input multiple-output (MIMO) and two-way relay can improve the transmission efficiency, while by setting the down tilt at the relay, the signal power and the transmission rate as well can be increased. The capacity of the massive MIMO two-way relay system is analyzed, and the lower-bound expression of the capacity thus calculated. In accordance with the distribution of the users, the lower-bound expectation is derived, with which the optimal searched and acquired down-tilt. As compared with the traditional approaches that set the down-tilt as the angle between the base station and the relay or the average angle of the users, the proposed approach can achieve a better balance of between the signal power at the base station and the signal power at the users, and acquire even higher lower bound of the system capacity. Simulations indicate that the proposed approach could achieve fairly high lower-bound of the system capacity.%大规模多输入多输出(MIMO)和双向中继可以有效提高传输效率,而通过在中继设置下倾角,可以提高信号功率和传输速率.因此,分析大规模MIMO双向中继系统的容量,获得容量下界表达式;根据用户的分布计算容量下界期望,进而搜索获得最优的下倾角.相比将中继倾角设置为中继和基站间夹角或用户的平均角度的方案,所提方案可以在基站信号功率和用户信号功率间取得最优平衡,获得更高的系统容量下界.仿真结果表明,所提的下倾角设置方案能够实现更高的容量下界.
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